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Fuel flow anomaly at higher altitudes

M

McSteff

Member
It has been bugging for me a while and I have found one old post ( http://www.vansairforce.com/community/showthread.php?t=88963
on this altitude related FF subject:
my fuel flow indication does not read accurately with increasing altitude. I am pretty happy with my k-factor of 71000 and the flow reads correctly approx. 4.5 gal at 5200RPM at lower altitudes. See graphs attached of a longer cross country flight at various altitudes:

Top chart : FF and DA
Bottom chart: FF and RPM

537ddt.png


Measuring the fuel quantity with a dip stick shows an avg of 4.5gal burnt per hr post flight.

Appreciate your help.
 
I am not sure what you mean?

AT higher altitude the fuel flow WILL be less. The carbs are pressure compensating so the flow will reduce due to lower DA.

I would also think as the float changes up and down at times the fuel flow at any given moment could be up/down due to flow surges at low demand?
 
Does the electric fuel pump run continuously or do you shut it off during cruise? I am thinking that maybe the fuel forms little vapor bubbles at higher altitude and that causes the flow meter to read incorrectly. But that is just a wild guess.
 
I don't have a separate switch for the elec fuel pump, it runs continuously. As per the graph the flow rate peaks at 7500 feet but then drops slightly at 9500 feet. But still way above what I would expect at such altitude (the POH states values below 4 GALs).

Seems like not many other users have come across this phenomenon, so I'll also touch base with Dynon.
 
we beat this to death a couple of years ago

but at density altitudes somewhere around 6500 feet the fuel flow reading does indeed go very high. Postflight checks show the fuel burn is as usual, but the indication is very definitely wrong. Nobody ever came up with an answer, but it's probably something ion the Dynon system. A case of too much information!

Wayne 120241/143WM 311 hours
 
After about six months of cruising at 5100 RPM and getting 4.5 GPH I set my K-Factor to show 4.5 GPH while on a trip. In a 3 HR flight it is now within 0.3 gallons of actual fuel used.
 
RFSchaller said:
After about six months of cruising at 5100 RPM and getting 4.5 GPH I set my K-Factor to show 4.5 GPH while on a trip. In a 3 HR flight it is now within 0.3 gallons of actual fuel used.
Click to expand...

Regarding the altitude, I'm interested in the answer to that question also.

Additionally, did you reset your k-factor while in flight, to 4.5 gph or did you figure a predetermined computation and reset it on the ground?

I still have to do it to my unit. I plan on setting it to 4.5 gph, at 5100 RPM, while in flight, at 2500 MSL (my most frequent cruise altitude in Florida). After that, I will make a couple long cross country flights and fine tune the setting. But I think that is a good place to start. Anybody concur with that?

Any ideas or advice from those who have done this before me is much appreciated.

The fuel flow computer on SkyView is a great fuel management tool to assist in determining the fuel status during cross country flying. I intend to use it to verify what I am seeing on my sight tubes.:)
 
Vans provided the following response on the FF issue:

"It is a known issue - but we don't have a fix for it yet. With the G3X system in the RV-12, we do not use the Fuel FLow feature, as with the current fuel system it can't be made truly accurate under all conditions. With 20/20 hindsight, the smart thing to do would have been to leave the sensor out of the fuel system, and save customers $200.

We have conducted a number of tests with various mods, but do not yet have a solution. The tank float gauge, if correctly calibrated, will give you the most accurate level of fuel remaining."

In a follow up email, they told me that even a second transducer in the return line will not provide perfect results either due to the always on pulsating elec fuel pump.
 
Last week I flew from La Center, WA to McCall, ID at 9500' and observed the same fluctuation in fuel flow at 5200 rpm. The fuel flow went from 5 gph to over 7 gph and then gradually came down after about 2 hrs in flight at the same altitude and rpm to 4.8 gph. Actual fuel used during the flight start to stop was 5 gph. Gallons of fuel used indication did not match the gallons remaining indication on the D180. As previously stated, the in tank fuel gauge I installed was accurate.

It's nice to know the background of the inaccuracy and that there's no fix.
 
Without a fuel flow indication we wouldn't worry about this. I have been blissfully flying for 30 years without all this info! Maybe I'm a dinosaur, but somehow I survived a lot of hours VFR and IFR.😀
 
rv3flier said:
?..........It's nice to know the background of the inaccuracy and that there's no fix.
Click to expand...

It is true, no one so far has identified the root cause of this behavior, despite work by Van's, Dynon, and several of us early builders. However, there is a work-around, and that is to shut off the electric pump except for T/O and landing, just like we used to do when flying Pipers and some other low-wing airplanes. Simple mod, add a switch to the pump circuit.
 
If you don't have Nav lights you don't even have to add a boost pump switch --- just wire the Nav light power to the electric fuel pump. Works great on my '12.
 
BigJohn said:
It is true, no one so far has identified the root cause of this behavior, despite work by Van's, Dynon, and several of us early builders. However, there is a work-around, and that is to shut off the electric pump except for T/O and landing, just like we used to do when flying Pipers and some other low-wing airplanes. Simple mod, add a switch to the pump circuit.
Click to expand...

Testing we have done while trying to isolate a cause showed that electric pump on or off has no influence.
 
rvbuilder2002 said:
Testing we have done while trying to isolate a cause showed that electric pump on or off has no influence.
Click to expand...

Good Morning Scott,

Don't know what testing you have done, what altitudes, etc., but all I can tell you is the testing I did back when I first noticed this behavior (2012-ish) very clearly proved that in my airplane turning off the electric pump always cleared the problem. There was a lot of chatter at that time that can be found on old VAF threads. If memory serves, Mitch Locke observed the same behavior in the blue factory plane, both with the D-180 and after conversion to Skyview. Just last week I had the same indication of high fuel flow at about 4 or 5 thousand MSL when I had forgotten to turn off the pump after climb out. Turned the pump off, fuel flow indication dropped right back to normal.

For newer builders and owners, please be aware that when this anomaly occurs, it is not an actual flow increase, only the indication of high flow.

Scott, I am foregoing my usual trip to OSH this year. Hope you have a successful show.
John
 
Last edited:
BigJohn said:
Good Morning Scott,

Don't know what testing you have done, what altitudes, etc.,

Scott, I am foregoing my usual trip to OSH this year.
Click to expand...


John, I may have mis spoken... Mitch has done most of the flight testing for this and would probably remember specifics better. Maybe he will comment.

I won't be at OSH either. I just finished a 2 week marithon painting and finishing the new RV-14, so I am going to take some vacation time... And try to paint my own airplane in 9 days.....
 
According to Matt Dralle of Matronics, high fuel flow indications can be a result of turbulent flow that the electric pumps generate. To that end, he sells a Matronics fuel flow dampener bulb. I bought one years ago but never installed it, but it's supposed to rectify that. Has Dynon ever tried something like that?
 
rv6n6r said:
According to Matt Dralle of Matronics, high fuel flow indications can be a result of turbulent flow that the electric pumps generate. To that end, he sells a Matronics fuel flow dampener bulb. I bought one years ago but never installed it, but it's supposed to rectify that. Has Dynon ever tried something like that?
Click to expand...

VERY interesting. This would be a great experiment. Install one of these in the tunnel just before the red cube. I may order one for a future "spare time" project. Cheap fix if it works. Scott, what do you think? (And, how is your painting going?)
 
BigJohn said:
VERY interesting. This would be a great experiment. Install one of these in the tunnel just before the red cube. I may order one for a future "spare time" project. Cheap fix if it works. Scott, what do you think? (And, how is your painting going?)
Click to expand...

This was already tried by Mitch on the blue RV-12 demonstrator and it made no difference.

Painting is going well... hopefully do reassembly tomorrow.
 
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